It was Plus mode and the only time it saved was the automatic save after the end credits. Nope, this achievement didn't unlock anything special, but it did have an amusing unexpected result.
When I got back to the file select, it said "None - 100%". Okay, so that's odd enough. I supposed when I selected it, I'd be back at the beginning, but with 100% completion, would that mean I retained all the abilities and whatnot?
Well, it did start up on the opening screen, in that dream sequence with the purple starry background...but the actual cutscene didn't happen. The Cosmic Owl didn't show up and I could move around freely. I had all the abilities from the end game, but the touchscreen was blank because BMO had yet to join the party. There was no sound at all except the pause beep, and there's nowhere to go beyond that one screen. So basically the Plus mode on that file is broken because I had the bright idea to do a no-save run. Heh.
I'm a little curious about doing a run where I only save at the first fountain now. I figure after beating it, I would start over at that fountain with all the abilities retained, but how much of the game's progress would be remembered? Would it be like I just came there from Ice King's Palace at the end, or would it really be like the start of the game and I could encounter all the bosses and cutscenes again? What sort of sequence-breaking shenanigans could I get into?
I reccommend other people experiment with this a bit. --- The Other Thundergod
Okay. My approach isn't about the challenge aspect though, I just wanted to see what would happen. I wasn't avoiding items or level-ups at any rate.
Besides just seeing what would happen without saving, I also wanted to see how fast I could beat it. My best time so far is about 80 minutes, and I don't think there's much else I can do to improve that short of stocking up on Habaneros and Baby Shoes in Normal Mode before starting Plus Mode. --- The Other Thundergod
Finally tried that thing where I only save at the beginning. The result was none of the interesting stuff I imagined. You just load at that first save point with all the progress up to the ending. No unopened chests, no reset cutscenes or bosses or locked doors or character placements. It makes sense. I guess I was just hoping for something interesting beyond running around the dreamscape.
I did find out one other thing about the dream room though. The invisible walls at the edge are "softer" than the ones everywhere else in the game. Doing Jake Roll into them you can get past the boundary and walk off the edge and disappear. It got me trying to roll off the edge of any other invisible walls in the game just to see if they left any others like that, but I couldn't find any. --- The Other Thundergod